A dielectric resonant element includes a dielectric block having a through hole, an outer conductor disposed on the outer surface of the dielectric block, and an inner conductor disposed on the inner surface of the through hole. A dielectric resonant element constitutes a λ/4 resonator, in which the inner conductor and the outer conductor are separated from each other at one end of the through hole and short-circuited at the other end of the through hole, or a λ/2 resonator, in which the inner conductor and the outer conductor are separated from each other at both ends of the through hole. The dielectric resonant element has a single-pair configuration that has only one pair of a through hole and an inner conductor (a dielectric resonant element having this configuration is hereinafter referred to as a discrete resonant element) or a multi-pair configuration that has multiple pairs of a through hole and an inner conductor (a dielectric resonant element having this configuration is hereinafter referred to as a bulk resonant element).
In a discrete resonant element constituting a λ/4 resonator, the Q factor may decrease due to concentration of electric currents around a short-circuited end at which the inner conductor and the outer conductor are short-circuited. In order to minimize concentration of electric currents, some discrete resonant elements have their outer conductors thickened around their short-circuited ends (see, for example, Japanese Unexamined Patent Application Publication No. 11-340713).
In the case where a filtering device includes a bulk resonant element having a multi-pair configuration, typically, resonators are coupled together using mutual capacitance between the inner conductors that occurs inside the dielectric blocks. On the other hand, in the case where a filtering device includes a discrete resonant element having a single-pair configuration, resonators are coupled together by inserting terminals from open ends into the through holes and by connecting together, using the terminals, the inner conductors and other circuit elements such as capacitive elements.
As described above, when a filtering device includes dielectric resonant elements and terminals, desired filter characteristics can be easily resulted from combining appropriate circuit elements. Such a filtering device can thus be designed highly flexibly.
Typically, a filtering device including a dielectric resonant element and a terminal has an electrode-free portion for separating the inner conductor and the outer conductor from each other on an end surface of the dielectric resonant element so as to prevent the inner conductor and the outer conductor from being electrically continuous with each other near the open end through a terminal or solder connected to the terminal. In such a filtering device, a portion of the surface of the dielectric resonant element is exposed without being covered with the outer conductor and thus the discrete resonant element is subject to external influences. Such a filtering device thus typically includes a shield member disposed so as to cover the discrete resonant element. In addition, in order to allow partial trimming of the outer conductor or the inner conductor of the discrete resonant element, the shield member is typically disposed in such a manner as to leave part of the discrete resonant element exposed.
In recent years, the radio communication band has been shifting to higher frequencies. Such a shift encourages the use of a frequency band unused thus far, such as a 5-GHz band, for radio communications. Accordingly, filtering devices including a dielectric resonant element and a terminal have been required to be adapted for higher frequency bands.
However, in order for filtering devices including a dielectric resonant element and a terminal to be adapted for higher frequency bands, the dielectric resonant elements are required to have a much smaller size than existing elements. Reduction of the size of the dielectric resonant elements renders it difficult to form circuit elements and as a result to adapt the filtering devices to higher frequency bands.
For example, if discrete resonant elements constituting λ/4 resonators according to Japanese Unexamined Patent Application Publication No. 11-340713 are reduced in size to a large extent, the capacitance required as a coupling capacitance for coupling the dielectric resonant elements together is significantly reduced. In order to fulfill a required coupling capacitance, the electrode area of a circuit element (capacitive element) connected to the terminal is required to be miniaturized or a drastic design change such as use of special material is required. Such a requirement hampers forming of a circuit element (capacitive element) into a general or simple configuration.
For example, significant size reduction of a discrete resonant element constituting a λ/4 resonator according to Japanese Unexamined Patent Application Publication No. 11-340713 requires size reduction of the shield member. This size reduction renders it difficult to leave the dielectric resonant element to be exposed, thereby impeding an operation of adjusting the filter characteristics of the filtering device.